In general, the process of preparing terephthalic acid comprises an oxidation step of oxidizing p-xylene with air in the presence of a catalyst such as cobalt, manganese and bromide and a distillation step of recovering the acetic acid solvent from the reactor and removing water.
Typically, conventional distillation, azeotropic distillation, etc. are utilized to separate and collect acetic acid from water.
FIG. 1 illustrates the conventional process of collecting acetic acid through azeotropic distillation using an azeotropic agent.
Referring to the figure, the conventional apparatus for collecting acetic acid through azeotropic distillation using an azeotropic agent comprises a dehydration tower (1) for separating acetic acid from water through azeotropic distillation, a condenser (2) for condensing the outlet gas from the top of the dehydration tower (1), an organic-water separation tank (3) for separating the liquid organic materials passing through the condenser (2) from water, a heater (4) for supplying steam to the dehydration tower (1) and a heat exchanger (5) for cooling the acetic acid discharged at the bottom of the dehydration tower.
This conventional technology is advantageous in that, by adding an azeotropic agent to a mixture of water and acetic acid, the energy consumption by the dehydration tower (1) can be reduced since the resultant azeotrope boils at a temperature lower than the boiling point of water.
However, because the supply of steam is needed to collect acetic acid, additional energy is required to remove water.
The reactor outlet gas formed during the preparation of terephthalic acid is hot, 180° C. or higher, and includes non-compressible gases, e.g., nitrogen, acetic acid, p-xylene and water.
The reactor outlet gas is passed through several heat exchangers containing cooling water for heat exchange in order to gradually lower the temperature of the reactor outlet gas. Condensed acetic acid and some of water are returned to the reactor and the remaining water is sent to the dehydration tower for discharging.
The gas that has passed through the final heat exchanger includes a small amount of acetic acid and p-xylene. The gas is sent to a high-pressure absorption tower, where p-xylene is collected as entrained by acetic acid, acetic acid is collected as entrained by water and non-condensible gases including nitrogen are sent to a gas discharging unit and processed there.
The liquid mixture of acetic acid and water that has passed through the final heat exchanger and has been condensed is sent to the dehydration tower, where acetic acid is discharged at the bottom of the dehydration tower and water is discharged at the top of the dehydration tower.